Techniques for forming isolation structures

ABSTRACT

Various techniques of forming isolation structures between adjacent diffusion regions are disclosed. In one technique, a thermally-grown oxide isolation structure is gouged out, and subsequent poly extending into the gouged out isolation structure is substantially level with the diffusion region. In another technique, a trench (bathtub) is formed, lightly oxidized, overfilled with polysilicon or amorphous silicon, gouged out, and thermally treated to form a substantially planar isolation structure. In another technique, an isolation structure exhibiting bird&#39;s-heads and bird&#39;s-beaks is polished until the bird&#39;s-beaks are removed. Gouging of the diffusion area may be permitted to occur. A bipolar transistor structure can be formed in the gouged diffusion area, and will exhibit reduced spacing between the intrinsic base and collector without proportionally reduced spacing between the extrinsic base and the collector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.07/748,853 filed Aug. 22, 1991, which is a continuation-in-part ofcommonly-owned, copending U.S. patent application Ser. No. 711,624,entitled TRENCH PLANARIZATION TECHNIQUES and filed on Jun. 6, 1991 bySchoenborn and Pasch.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to polishing techniques for semiconductordevices, more particularly to the polishing of field (isolation) oxideregions between (i.e., surrounding) active (diffusion) regions.

BACKGROUND OF THE INVENTION

Electrical isolation of semiconductor integrated transistors from oneanother can be achieved by laterally (in the plane of the wafer)isolating "active" regions of the device with insulating material. Twotechniques are common: 1) selectively oxidizing wafer siliconsurrounding the active region (known as "LOCOS", or Local Oxidation ofSilicon), or 2) depositing insulating material, such as silicon dioxide("oxide") in a trench (or "bathtub") around the active region. Theformer, selectively oxidizing to form a field (isolation) oxide region,is discussed in the main hereinafter.

One known technique of forming isolation oxide around a diffusion areais Local Oxidation Of Silicon ("LOCOS"). In the typical LOCOS process, amask (e.g., nitride) is applied to the wafer, a trench is etched, thewafer is heated, and oxide "grows" predominantly in the trench. In thismanner, an isolation structure is created that extends both into thewafer and some height above the wafer. In a "semi-recessed" version ofthis process, the height of the oxide structure above wafer level isapproximately 45% of the total thickness of the as-grown oxide. In a"fully-sunk" ("fully-recessed") version of this process, the isolationstructure can grow to a height of about 25-45% of the total oxidethickness, above wafer level. No matter how it is grown, the resultingoxide structure has a prominent portion above wafer level, resulting inan irregular top surface wafer topography. It is known to polish thewafer to remove the prominent portion of the isolation structure, butthis usually involves steps ensuring that the isolation oxide structuredoes not become gouged out below wafer level, especially if a polishstop (e.g., nitride cap or mask layer) is employed to protect thediffusion region.

LOCOS is described in U.S. Pat. Nos. 4,897,364, 4,903,109 and 4,927,780,incorporated by reference herein.

Subsequent deposition of polysilicon ("poly"), which may typicallyfollow the LOCOS process, usually places poly on top of the diffusionareas and on top of the LOCOS oxide. The top of the poly is typically3000-4000 Å above the level of the diffusion area, simply because of thenormal poly thickness. The poly that is located above the LOCOS regionis another 4000-5000 Å above the level of the top of the poly over thediffusion region, simply because of the prominence (not polished) of theoxide in the LOCOS area. For this reason, the difference in heightbetween the diffusion area and the poly over the LOCOS area can be asgreat as 8000 Å. This large difference in height is very undesirable,but is the natural consequence of the process as currently implemented.

FIG. 1 graphically illustrates the situation, and shows a semiconductordevice 110 having a silicon wafer 112, a diffusion area (active region)114 and field oxide areas (e.g., LOCOS) 116 adjacent the diffusionregion 114. Inasmuch as the field oxide areas 116 are thermally formed,they exhibit a raised topography at the wafer surface. U.S. Pat. Nos.4,892,845, 4,897,150, 4,918,510, 4,935,378, 4,935,804, 4,954,214,4,954,459 and 4,966,861 illustrate structures of this general type, andare incorporated by reference herein.

A polysilicon layer 118 is deposited over the diffusion area 114, andextends from the diffusion region 114 to at least partially over theadjacent field oxide areas 116. The polysilicon layer is shownsegmented, discontinuous at the center of the diffusion region 114, butis can be contiguous and extend entirely over the diffusion region. Anoverlying, generally-conformal insulating oxide layer 120 is depositedover the wafer. Vias 122a and 122b are formed through the insulatingoxide 120--one via 122a for making contact with the diffusion area 114at wafer level, and another via 122b for making contact with the poly118 over the field oxide area 116. Evidently, the vias 122a and 122b areof unequal depth (even if the insulating layer 120 is subsequentlyplanarized), which causes problems with subsequent via-filling. Asmentioned hereinabove, the via 122b to the poly over the field oxidearea is shallower by the height (above wafer level) of the field oxidearea plus the poly thickness. The problems associated with filling viasof unequal depth are discussed described in commonly-owned U.S. Pat.Nos. 4,708,770 and 4,879,257, incorporated by reference herein.

Another problem is that the top surface of the insulating layer 120 ishighly irregular (not smooth and non-planar). This irregular top surfacetopography will propagate through subsequent depositions, if leftunchecked, making subsequent processing steps more complicated (e.g.,requiring a planarization step).

As mentioned above, planarizing the isolation and diffusion regions(e.g., by polishing back the oxide prominence) is complicated by thedifferent hardnesses of the isolation oxide and the (essentiallysilicon) diffusion region. The diffusion region, essentially nativesilicon, is softer than oxide (SiO₂), but a polishing stop can beincorporated over the diffusion region. In either case, materialhardness differences are the problem.

Consider, for example, the case of a "fully-sunken" LOCOS oxidestructure exhibiting a "Bird's-Beak", as shown in FIG. 4A. In thetypical implementation of polishing the LOCOS structure (to remove the"Bird's-Head"), the polishing is stopped before the Bird's Beak istotally removed. This is objectionable, because the Bird's-Beak extendslaterally into the diffusion area in the polished structure, as shown inFIG. 4B. The bird's-beak is one of the most objectionable aspects ofconventional dielectric isolation schemes, and a polish technique thatdoes not effectively remove this structure, without resorting todeposited dielectric films, is lacking in utility.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide improved techniquesfor forming isolation structures.

It is another object of the present invention to provide a technique forreducing the difference in height between the diffusion area andsubsequent poly over the field oxide area, thereby resulting in: a)substantially simpler subsequent creation of contacts through anoverlying insulating layer; b) a smoother, more planar top surfacetopography of the overlying insulating layer; and c) substantiallysimpler subsequent deposition of metal over the overlying insulatinglayer.

It is another object of the present invention to provide a technique forcreating an isolation structure which is substantially planar withregard to the diffusion region, and which polishes substantially fasterthan normal thermally-grown field oxide isolation regions.

It is another object of the present invention to provide a technique forusing conventional LOCOS dielectric oxidation isolation structureprocesses, and simultaneously retaining the advantages of planarizationby polishing.

It is another object of the present invention to provide an improvedbipolar transistor structure, benefiting from the techniques disclosedherein.

According to the invention, an isolation structure between diffusionregions is polished (e.g., using chem-mech polishing techniques) untilit is gouged out, below wafer level. A subsequent layer, such aspolysilicon, extending over the edge of the diffusion regions anddrooping into the gouged out isolation structure exhibits increaseduniformity in height above wafer level. This is useful, inter alia, inthe filling of vias formed through a subsequent insulating layer.

Further according to the invention, polysilicon or amorphous silicon isused to form an isolation structure. A bathtub (isolation trench) isetched and overfilled with a material selected from the group ofpolysilicon or amorphous silicon. Polishing proceeds until the isolationstructure (within the bathtub) is gouged out below wafer level. Then,the isolation structure is thermally oxidized, and expands to form asubstantially planar isolation structure. A thin oxide layer may beformed in the bathtub prior to depositing the polysilicon (or amorphoussilicon).

Further according to the invention, an isolation oxide structure, suchas a fully-sunk (fully-recessed) oxide structure exhibiting abird's-head and bird's-beaks, is polished, with polishing continuinguntil essentially or actually all of the bird's-beak is removed withpolishing. In order for this technique to be effective, the isolationoxide is grown to a greater thickness than normal. Gouging in thediffusion area is small, and can be controlled such as by nitridecapping.

Further according to the invention, an isolation structure, such as afully-sunk (fully-recessed) oxide structure exhibiting a bird's-head andbird's-beaks, is polished, with polishing continuing until essentiallyor actually all of the bird's-beak is removed with polishing. Thediffusion area is permitted to be gouged (e.g., no polish stop isincorporated). This allows for bipolar transistor structures to beformed in the diffusion area with reduced collector to (intrinsic) basespacing, and without significantly reduced collector to (extrinsic) basespacing.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art semiconductor device.

FIG. 2A is a cross-sectional view of an in-process semiconductor deviceformed by the technique of the present invention.

FIG. 2B is a cross-sectional view of the semiconductor device of FIG.2A, at a later stage in fabrication.

FIG. 3A is a cross-sectional view of an in-process semiconductorstructure, according to another embodiment of the invention.

FIG. 3B is a cross-sectional view of the semiconductor structure of FIG.3A, at a later stage of fabrication.

FIG. 3C is a cross-sectional view of the semiconductor structure of FIG.3B, at a later stage of fabrication.

FIG. 3D is a cross-sectional view of the semiconductor structure of FIG.3C, at a later stage of fabrication.

FIG. 4A is a cross-sectional view of a prior art LOCOS, fully-sunkisolation structure, exhibiting a bird's-head and bird's beaks.

FIG. 4B is a cross-sectional view of the isolation structure of FIG. 4A,after conventional polishing, according to the prior art.

FIG. 4C is a cross-sectional view of a semiconductor structureincorporating the isolation structure of FIG. 4A, after polishingaccording to the present invention.

FIG. 5A is a cross-sectional view of a "generic" bipolar transistor ofthe prior art.

FIG. 5B is a cross-sectional view of a bipolar semiconductor structure,according to the present invention, which is formed based on theinventive polishing technique illustrated in FIG. 4C.

Throughout all of the descriptions contained herein, while only onesemiconductor structure or isolation structure may be discussed, itshould be understood that the invention is applicable to semiconductordevices employing many such structures.

The present invention, in its various embodiments, benefits from thequantitative and qualitative understandings of polishing, as describedin the aforementioned U.S. patent application Ser. No. 711,624, entitledTRENCH PLANARIZATION TECHNIQUES and filed on Jun. 6, 1991 by Schoenbornand Pasch,

When polishing is referred to herein, it should be understood that itcan be abrasive polishing (lapping), as described in U.S. Pat. No.4,940,507, but is preferably chemi-mechanical (chemmech) polishing asdescribed in U.S. Pat. Nos. 4,671,851, 4,910,155, 4,944,836, all ofwhich patents are incorporated by reference herein. Whenchemi-mechanical polishing is referred to hereinafter, it should beunderstood to be performed with a suitable slurry, such as Cabot SC-1.

DETAILED DESCRIPTION OF THE INVENTION Gouged Field Oxides

FIG. 1, discussed hereinabove, illustrates the problem of vias extendingto poly over LOCOS regions being shallower than vias extending todiffusion regions (or to poly over the diffusion region).

FIG. 2A shows an in-process semiconductor device 210 formed according tothe present invention. The device 210 includes a substrate 212, in whichfield oxide (e.g., LOCOS) regions 216 are formed adjacent a diffusion(active) region 214. Typically, as illustrated, the LOCOS regions 216extend prominently above the top surface of the wafer 212, and create anirregular top surface topography. It is known to planarize irregularsemiconductor structures at various phases of fabrication by techniquessuch as abrasive polishing (e.g., lapping), chem-mech polishing andetching. In some cases, sacrificial layers are applied prior toplanarization.

Polishing to planarize a field oxide region is feasible, but tends toproceed at a faster rate with respect to the field oxide than withrespect to the capped diffusion area, thereby gouging the field softerfield oxide. As described in commonly-owned, copending U.S. Pat. No.711,624, entitled TRENCH PLANARIZATION TECHNIQUES and filed on Jun. 6,1991, various techniques can be employed to characterize, and thereforeimplement processes to avoid this seemingly undesirable result.

According to the present invention, it is advantageous to permit gougingthe field oxide 216 during polishing, which will be beneficial infurther processing steps.

FIG. 2B shows the semiconductor device 210 after polishing (abrasive orchem-mech polishing), wherein it has been permitted that the polishingprocess gouges the softer field oxide 216 so that its top surface isbelow wafer level (and consequently below the top surface of thediffusion area 214). The resulting gouged field oxide structure isdenoted as 216', and it can be seen that it is "dished" (bowl-shaped).At its inner and outer peripheries, it is at wafer level, and it slopessmoothly down (below wafer level) towards its center. Typical dimensionsfor this field oxide region are: its width (w) is about 2-5 microns, andits center is depressed a depth (d) about 3000-4000 Å below wafer level.Field oxide regions, generally, can be up to 50 microns wide.

Further illustrated in FIG. 2B, a layer 218, such as patterned poly, isdeposited and extends over the diffusion area 214 in at least an areaadjacent a gouged field oxide 216', and extends over the gouged fieldoxide 216' in at least an area adjacent a diffusion area 214. As in FIG.1, an insulating layer 220 is deposited over the entire device, and vias222a and 222b are formed through the insulating layer 220 for connectingto the diffusion area 214 and to the poly over the gouged field oxide216'.

Notably, the vias 222a and 222b are of substantially equal depth, due tothe gouging of the field oxide 216'. For example, if the field oxide216' is gouged to a depth (d) of 3000-4000 Å below wafer level, and thethickness of the poly 218 over the gouged field oxide 216' is 3000-4000Å thick, then the top surface of the poly over the gouged field oxide216' will be substantially at wafer level--in other words, level withthe top surface of the diffusion area 214.

Further, as a result of gouging the field oxide, the top surfacetopography of the insulating layer 220 is smoother and more planar(i.e., than shown in FIG. 1, which illustrates the irregular topographyof the prior art).

In other words, as shown in FIG. 2B, the polysilicon which extends overthe edge of the diffusion region 214 will "droop" down into the gougedout area of the field oxide region 216'. If the gouging is not toogreat, (e.g., less than the thickness of the polysilicon below the levelof the diffusion area), the difference in position of the diffusionarea, the top of the polysilicon in the diffusion area, and the top ofthe polysilicon in the field oxide region will be reduced to only thethickness of the polysilicon. If the gouging is substantially equal toor greater than the thickness of the poly, substantial equality can beachieved between the position of the diffusion area and the poly overoxide.

The significant improvement in the topography of the wafer allows for amore straightforward creation of a planarized deposited silicon dioxide(220) forming the insulation to the first metal layer (not shown). Theimprovement in the smoothness of the insulating layer can have numerousbeneficial effects on the subsequent processing of wafers and on theeventual yield and reliability of the wafer.

While it has been shown that the poly layer (118 of FIG. 1 and 218 ofFIGS. 2A and 2B) is segmented, having an opening above the diffusionarea (114,214), the poly layer 118, 218) can also extend fully acrossthe diffusion area (114, 214). In that case, a via formed through anoverlying insulating layer (120,220) can also extend through the polylayer (118,218). The teachings of the present invention are equallyapplicable in that case.

Surprisingly, what was previously thought to be an undesirable processside effect (gouging of oxides during polishing) serves as the basis ofan improved process. An oxide isolation structure is created whichautomatically places overlying poly structures in a more advantageousposition for subsequent contact formation.

The use of silicon-on-sapphire technology gives somewhat similaradvantages, but the complexity and cost are prohibitive. Processes usingnitride-guarded sidewalls, such as "SWAMI", for recessed oxides do notas a rule give smooth transitions between the diffusion area and theaccompanying field oxide region. Also, these technologies are morecomplex than the disclosed process.

Oxidized Polysilicon Isolation

FIGS. 3A-3D illustrate a technique for forming substantially planarisolation structures. It will be evident that the process involves somesteps resembling those used in the formation of Recessed Oxide Isolation(ROI) techniques, some resembling those used in thermal oxidetechniques, as well as some of the techniques described above withrespect to FIGS. 2A and 2B.

FIG. 3A shows an in-process semiconductor device 310 having a wafer 312.A trench, or "bathtub" 316 is etched into the wafer, in an area whichwill become the isolation region, and has a depth "h". Diffusion regions314 are adjacent the inchoate isolation region 316. This stage of theprocess resembles steps employed in the formation of ROI trenchisolation structures, but at this point the process diverges from theteachings of the prior art.

FIG. 3B shows the next step in the process. The bathtub 316 ispreferably lightly oxidized using conventional thermal oxidationprocesses to grow a very thin layer 318 of thermal oxide in the bathtub,and optionally over the diffusion region (if not capped). By "verythin", it is meant that the thermal oxide layer 318 would be on theorder of 400-600 Å, which is significantly less than the depth of thebathtub (and eventual field oxide thickness) which will be on the orderof 1500-2000 Å, by way of example. This thin layer of thermal oxide ispreferably formed in the bathtub to serve as a stress relief layer forsubsequent deposition.

A layer of material 320 selected from the group of amorphous silicon orpolysilicon materials is deposited over the thermal oxide layer 318. Thelayer 320 is deposited to a thickness that is greater than the depth "h"of the bathtub 316, and exhibits a depression (trough) 322 above thebathtub 316. The layer 320 is now polished.

FIG. 3C shows the layer 320 after it has been polished. A portion 320aof the layer 320 is within the bathtub 316, and is intentionallypolished until it is gouged, or depressed below wafer level. Itsperipheral edges are substantially at wafer level, and its center issubstantially below wafer level. There may also be a thin remnant of thelayer 320 remaining over the diffusion areas 314, but it is not shown.In order to control this polishing process, the teachings of theaforementioned U.S. patent application Ser. No. 711,624, entitled TRENCHPLANARIZATION TECHNIQUES and filed by Schoenborn and Pasch, areespecially helpful.

At this point, the layer 320a within the bathtub 316 looks like a layerof oxide, but herein the desirability of the use of polysilicon (oramorphous silicon) is revealed.

As shown in FIG. 3C, the portion 320a within the bathtub 316 is thickestat the center of the bathtub 314, and thinner towards the periphery(edge) of the bathtub bordering the diffusion regions 316.

The polished polysilicon 320a (FIG. 3C) is now oxidized in a thermaloxidation furnace. Because of the expansion of polysilicon (or amorphoussilicon) upon oxidation, significant changes occur to the topography ofthe portion 320a. The gouged out area expands (grows) upward, forming asubstantially flat isolation structure 324 within the bathtub 316. Thisis because the structure 320a expands more at its center, where it isthicker, than at its periphery adjacent the diffusion regions 314, whereit is thinner. This results in an isolation structure 324, shown in FIG.3D, that is substantially planar across its entire surface, afterthermal oxidation.

The use of polysilicon (or amorphous silicon) for isolation in themanner described above yields beneficial results in the formation of areasonably planar isolation structure. As mentioned above, the processbenefits from various steps that are known in the formation of ROIisolation structures, in the formation of LTO isolation structures, andin the gouging of isolation structures (described hereinabove), with thenon-obvious selection of materials for the layer 320.

The invention solves the conventional problem of significant irregulartopography of isolation structures, while also solving the problem ofgouging the dielectric film inherent in conventional polishingprocesses. Current processes such as fully-recessed oxide, "SWAMI", and"zero Bird's Head are complicated, and often involve compromises in thequality of the isolation to diffusion interface. The disclosed techniquemakes fewer compromises to arrive at an excellent topography, andexhibits less stress at the isolation to diffusion interface. Thisprovides a possibility of improving the device packing density, using aprocess of good productivity and producing adequate device performance.

Although not shown, if polishing proceeds to the point where a layer ofpolysilicon (or amorphous silicon) remains over the diffusion regions,the remnant over the diffusion regions may be used in the formation ofsubsequent structures, as desired.

Removing Bird's-Heads and Bird's-Beaks

FIGS. 4A and 4B illustrate a prior art technique for creating a LOCOSfully-sunk isolation oxide structure. As shown in FIG. 4A, asemiconductor device 410 has a wafer 412, and a thermally-grown fieldoxide structure 416 adjacent a diffusion area 414. The oxide structure416 exhibits bird's-heads 418 and bird's-beaks 420, both adjacent thediffusion areas 414. Bird's-head structures, which are a normalconsequence of LOCOS oxidations, are objectionable because they areespecially non-planar, and bird's-beaks are objectionable because theyintrude upon a significant portion of the diffusion regions, makingthose portions of the diffusion regions virtually unusable.

FIG. 4B shows the structure after polishing, according to the prior art,and it is evident that the bird's-beaks 420 remain after polishing. Thisis the situation illustrated in U.S. Pat. No. 4,671,851, entitled METHODFOR REMOVING PROTUBERANCES AT THE SURFACE OF A SEMICONDUCTOR WAFER USINGCHEM-MECH POLISHING TECHNIQUE (Beyer, et al.; 1987).

Bird's-Beaks are described in U.S. Pat. Nos. 4,897,365, 4,912,062,4,952,525 and 4,959,325, incorporated by reference herein.

According to the present invention, it is possible to use a conventionalLOCOS dielectric oxidation isolation structure process, andsimultaneously retain the advantages of planarization by polishing. Thetechnique requires that the polishing of the isolation structure becontinued until the dielectric film over the diffusion region has beencompletely removed, as shown in FIG. 4B. Polishing is then continueduntil essentially or actually all of the bird's-beak 420 is removed withpolishing.

The resulting structure is shown in FIG. 4C. Evidently, the bird's-beaks420 have been removed, and there is only a minor gouging of thediffusion regions 414. A reference plane "P" is shown by a dashed linein FIGS. 4B and 4C, to illustrate how far polishing must proceed toaccomplish the purpose of completely removing the bird's-beaks 420.

The process requires a minor change in the growth of the field oxide forisolation. Preferably the use of a fully-recessed oxide isolation isgrown to a somewhat greater thickness than is normally done, such as afew hundred Angstroms greater thickness (i.e., grown to approximately10% greater thickness than normal). The optimal amount of excess growthis driven by the overall parameters of the particular structure, and canbe determined empirically. To some extent, less extra growth results inmore efficiency, and the ten percent figure set forth above is nominallyoptimal.

The combination of the fully sunk oxide isolation and its greaterthickness is required to compensate for the material being removedduring polishing. (Evidently, as illustrated in FIG. 4C, the trenchdepth, and hence the oxide thickness in the completed structure has beendiminished by the advertent over-polishing.)

This procedure makes the polishing create a structure that has a sharpedge between the diffusion region 414 and the isolation region 416, andthere are no bird's-beaks to accommodate in subsequent processing steps.

Because the diffusion area is made of softer silicon crystal material,the diffusion area is expected to polish must faster than thesurrounding silicon dioxide isolation area. This phenomenon could beadvantageous, depending on the semiconductor process being implemented.However, if gouging the diffusion area is deemed undesirable for aparticular process, it may be capped with a polish stop, such as siliconnitride. (The silicon nitride cap can simply be the mask used foretching the isolation trench, and it can be retained to prevent gougingthe diffusion region.) An appropriately chosen nitride thickness wouldlargely eliminate any tendency for the polishing to gouge out thediffusion area.

The technique of this invention provides a method of creating anisolation structure which has a smaller than normal transition fromdiffusion to the oxide material. Further, by effective removal of thebird's-head and bird's-beak structures, more area is available foractive elements, and denser circuits are possible.

Prior art polishing techniques (e.g., for removing bird's-heads)typically require CVD deposited dielectric films, which sometimes havequestionable electrical characteristics.

This method gives a sharp isolation transition without the problems ofstress concentration common with previous methods.

BIPOLAR TRANSISTOR

As illustrated above, in FIGS. 4A-4C, it is possible to polish isolationstructures and create substantially planar isolation and diffusionstructures. Also, it was noted that the diffusion area can (if notcapped, or not adequately capped) take on a perceptible depression inthe middle of the diffusion area after polish. Herein is disclosed theformation of a subsequent structure, a bipolar transistor, whichbenefits from allowing the diffusion region to be depressed (gouged).

FIG. 5A shows a "generic" bipolar transistor structure 510 of the priorart. A wafer 512 has a diffusion region 514 surrounded by two isolationregions 516. A collector structure 518 is buried within the diffusionregion, according to known techniques. An emitter structure 520 isformed atop the diffusion region, according to known techniques. Anintrinsic base structure 522 is also formed within the diffusion region,between the emitter and the collector at a given distance from the topsurface of the periphery of the diffusion region, according to knowntechniques. An extrinsic base 524, or "base sink" is formed just withinthe top surface of the diffusion region, according to known techniques.

As shown in FIG. 5A, the top surface of the diffusion region 514 tendsto be "crowned" (i.e., opposite of "gouged"). Hence there is asignificant distance between the intrinsic base 52 and the collector518.

According to the present invention, it is desirable to reduce thespacing between the intrinsic base (522) and the collector (518),without proportionally decreasing the distance between the extrinsicbase (524) and the collector (518).

FIG. 5B shows a bipolar transistor structure 510', fabricated accordingto the present invention. Referring back to FIG. 4C, and the discussionthereof, it was described how the diffusion region 514' could be allowedto become gouged (depressed within the wafer 512'), by polishing withouta polish stop. (This accounts for the somewhat more prominent fieldoxide regions 516' in FIG. 5B than were evident in FIG. 4C.)

As is evident in FIG. 5B, because the diffusion region is gouged, thespacing between the intrinsic base 522' and the collector 518' isreduced, since the intrinsic base 522' is formed at a given distancebelow the depressed center of the diffusion region 514'. A decrease inintrinsic base to collector spacing on the order of 0.1-0.3 microns, ormore, can be achieved, and is very desirable to the basic transistorarchitecture.

At the same time, the spacing between the extrinsic base 524' and thecollector 518' has not been proportionally reduced, since the extrinsicbase resides at the periphery of the diffusion region (i.e., upper edgeof the bowl), which is virtually at wafer level. This is significantbecause it would be undesirable to decrease the spacing between theextrinsic base and the collector, which would disadvantageously increasethe (extrinsic) base-to-collector capacitance.

In other words, all other process factors being equal (depth ofcollector below wafer level, depth of intrinsic base below top ofdiffusion region, position and penetration of extrinsic base), bydepressing (gouging) the diffusion region, several advantages to thetransistor architecture (and performance) can be derived.

What is claimed is:
 1. A method of forming a bipolar transistor structure, comprising:on a silicon wafer, thermally forming at least one isolation structure adjacent at least one diffusion area, wherein the diffusion region is substantially at wafer level and the isolation structure extends above wafer level; polishing back the isolation structure, and permitting the diffusion area to become gouged and exhibit a depressed top surface topography; forming a collector within the diffusion area; forming an emitter atop the center of the diffusion area; forming an intrinsic base within the diffusion area between the emitter structure and the collector structure; forming an extrinsic base just within the top peripheral edge of the diffusion area; wherein: the spacing between the intrinsic base and collector is reduced without proportionally reducing the spacing between the extrinsic base and the collector.
 2. A method according to claim 1, wherein:the spacing between the intrinsic base and the collector is reduced on the order of 0.1-0.3 microns. 